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480                              A   FISH-EATING   RAT                              CHAR
pads,  a  state  of affairs  which characterises  a  number of these
Bodents.
The genera Megalomys, Chilomys, HeitTirodontomys, Eligmo-
dontia, Nectomys, Rhipidomys, Tylomys, Uolochilus, Heithrodon,
Phyttotis, Scapteromys, Acodon, Oxymycterus, Ichthyomys, Blari-
nomys, Notiomys are South American forms. Oryzomys and
Mheitkrodontomys are common to both parts of the Kew "World.
The genus IcJithyomys is remarkable on account of its un~
B-odent-like habits and of certain associated structural changes.
I. stolzmanni was obtained from Mount Chanchamays in Peru.
at an altitude of 3000 feet; it is a habitual fish-eater, and
lives in streams. Another species, I. Jiydrobates, was formerly
referred to Habrothrix. The skull shows likenesses to that of
the Australian Hydromys; but the most marked characters of
adaptation are those of the teeth and caecum. The cutting
edges of the upper incisors form a reversed V of obvious use
in holding a slippery fish. The caecum is much reduced, short,
and narrow. The general Otter-like shape of the creature is
largely due to its flattened head, though its " size and general
proportions are much as in the common Black B-at." *
This sub-family contains a number of genera from Madagascar,
viz. Br achy tarsomys, JWeso-mys, HallomySj Bra£hyuromys> jETypo-
geo-mys, Grym'nuroTnys, and lEliurus.
Sub-Fam. 10. Heotominae.—The last sub-family of the
Muridae is that of the Neotominae, containing the ^NTorth Ameri-
can genera NeQtom,a> ^Keno'nvys, Hodomys, and JVelsonia.
Fam. 3. Bathyergidae.—This family contains several genera
which consist of subterranean forms. All these Kodents agree
in a number of characters, of which the principal are as
follows:—
The eyes are very small, and the external ears are reduced to
the merest fringe of skin round the aural aperture. The legs are
short, as is the tail; the hair-covering is reduced—a reduction
which finds its culmination in the nearly nude IleterocepTiatus.
Being burrowing creatures, a number of other modifications in
accordance with this mode of life are to be seen in their
structure. The upper incisors stand out in front of the closed
lips, and prevent the entrance of earth. For the same reason
1 See O. Thomas,  "On some Mammals from Central Peru," Proc.  Zool* Soo.